One number is 3 times another number. If 15 is added to both numbers, then one of the new numbers becomes twice that of the other new number. Find the numbers. 

Let the number be x. 

Then the other number = 3x 

When 15 is added to both numbers then the new numbers are x + 15 and 3x + 15. 

Now according to the question:

=> 3x + 15 = 2(x +15) 

=> 3x + 15 = 2x + 30 

=> 3x - 2x =30 - 15 

=> x = 15 

And, 3x = 3 × 15 = 45 

Hence, the required numbers are 15 and 45.